This invention relates to a starch-based, alkaline corrugating adhesive composition which contains a selected crosslinking additive that has exceptional low levels of residual formaldehyde content and helps impart good water-resistance and viscosity stability to the adhesive.
The procedures employed in the production of corrugated paperboard usually involve a continuous process whereby a strip of paperboard is first corrugated by means of heated, fluted rolls. The protruding tips on one side of this fluted paperboard strip are then coated with an adhesive, and a flat sheet of paperboard, commonly known in the trade as a facing, is thereafter applied to these tips. By applying heat and pressure to the two paperboard strips thus brought together, an adhesive bond is formed therebetween. The above-described procedure produces what is known as a single-faced board in that the facing is applied to only one surface thereof. If a double-faced paperboard in which an inner fluted layer is sandwiched between two facings is desired, a second operation is performed wherein the adhesive is applied to the exposed tips of a single-faced board and the adhesive-coated tips are then pressed against a second facing in the combining section of the corrugator under the influence of pressure and heat. The typical corrugating process and the use and operation of corrugators in general are described in U.S. Pat. Nos. 2,051,025 and 2,102,937 to Bauer.
Starch-based adhesives are most commonly used in the corrugating process due to their desirable adhesive properties, low cost and ease of preparation.
The most fundamental of starch corrugating adhesives is an alkaline adhesive which is comprised of raw, ungelatinized starch suspended in an aqueous dispersion of cooked starch. The adhesive is produced by gelatinizing starch in water with sodium hydroxide (caustic soda) to yield a primary mix of gelatinized or cooked carrier, which is then slowly added to a secondary mix of raw (ungelatinized) starch, borax and water to produce the full-formulation adhesive. In the corrugating process, the adhesive is applied (usually at between 25xc2x0 and 55xc2x0 C.) to the tips of the fluted paper medium or single-faced board, whereupon the application of heat causes the raw starch to gelatinize, resulting in an instantaneous increase in viscosity and formation of the adhesive bond. Such adhesives are described in the above-noted patents to Bauer. Typical xe2x80x9cno carrierxe2x80x9d starch adhesives are described in U.S. Pat. No. 3,487,033 to McElmury et al., and U.S. Pat. No. 3,355,307 to Schoenberger et al.
It is often desired or necessary in the manufacture of corrugated paperboard that the adhesive yield water-resistant bonds which can withstand extended exposure to high humidity, liquid water, melting ice and the like. A number of approaches have been devised to produce water-resistant corrugating adhesives. One method involves the preparation of an acidic, starch-based adhesive wherein urea-formaldehyde resin is added to the composition, together with an acidic catalyst such as aluminum sulfate, to produce water-resistant bonds in the corrugated board manufactured therewith. The adhesive composition itself, however, is deficient in other important properties such as corrugator bonding speeds, viscosity stability and pot life, and exhibits excessive formaldehyde odor. In addition, acidic corrugating adhesives tend to be corrosive.
The many disadvantages associated with the acidic corrugating adhesives led to the development of water-resistant alkaline curing starch-based adhesives for use in the corrugating industry. In the preparation thereof, a thermosetting resin, such as, e.g., urea-formaldehyde, resorcinol-formaldehyde, melamine-formaldehyde, phenol-formaldehyde, diacetone acrylamide-formaldehyde, ketone-aldehyde and urea-acetone-formaldehyde condensate, is added to the adhesive as a crosslinking additive for the amylaceous components to produce water-resistant bonds. Preferred among these resins for superior water-resistant properties are ketone-formaldehyde condensates as disclosed in U.S. Pat. No. 2,529,851, and particularly acetone-formaldehyde resins. Some adhesives made from such resins, however, suffer from poor pot life and viscosity instability, as well as considerable formaldehyde odor.
In recent years, due to the toxicity of and increasing governmental regulations concerning formaldehyde, serious efforts have been made to reduce the levels of exposure to formaldehyde in the industrial workplace. Acetone-formaldehyde resins such as are employed as crosslinking additives in corrugating adhesives contain about 1.0 to 4.0% free (unreacted) formaldehyde by weight of condensate. Prior attempts to reduce formaldehyde levels in crosslinking additives as taught in U.S. Pat. Nos. 3,019,120 and 3,294,716 have not reduced free-formaldehyde amounts to a significant extent and/or have resulted in diminution of the degree of water-resistance achieved in the bonds formed.
In U.S. Pat. No. 4,366,275 to Silano et al., the crosslinking additive used with the starch-based alkaline corrugating composition comprises a mixture of acetone-formaldehyde condensate and dimethylol dihydroxy ethylene urea (DMDHEU) wherein at least a portion of the DMDHEU present is produced xe2x80x9cin situxe2x80x9d by reaction of the free-formaldehyde contained in the acetone-formaldehyde condensate with dihydroxy ethylene urea. The patent discloses that the unreacted formaldehyde in the acetone-formaldehyde resin condensate is reduced to about 0.1 to 2% by weight. Experience has shown, however, that in most instances the free-formaldehyde is reduced only to a level of about 0.5 to 0.9% by weight of the condensate. Current industry requirements call for still lower levels of unreacted formaldehyde.
A recent patent, U.S. Pat. No. 5,079,067 to Willging, discloses the reduction of free-formaldehyde in formaldehyde containing resins to a level of less than 0.3%, by weight (of aqueous resin composition), by reacting the free-formaldehyde with a nitrogen base and urea in the presence of an acid catalyst.
Another recent patent, U.S. Pat No. 5,247,066 to J. Schoenberg et al., discloses another method for reducing levels of free-formaldehyde in ketone-formaldehyde crosslinking additives by treating the unreacted formaldehyde with hydrogen peroxide. This method has resulted in significant reduction in free-formaldehyde content to levels of less than about 0.4% by weight of condensate (i.e., aqueous condensate or solution).
While these methods generally provide lower free-formaldehyde levels than previously attained, they do not always provide the water-resistance, viscosity characteristics and ease of process conditions that are desired. Furthermore, there is a need and desire to develop corrugating adhesive compositions which while providing suitable water-resistance, also have reduced levels of residual formaldehyde and particularly are viscosity stable.
It has long been known to use sulfite salts to reduce formaldehyde content in different non-related technical such as textiles, plywood/particle board manufacture, cosmetics and paper production. However, it has heretofore not been known to use ketone-formaldehyde additives treated with selected sulfite salts in corrugating adhesives to provide compositions with good water-resistance and particularly improved viscosity stability while also providing low levels of free formaldehyde content in the additive.
It has now been found that a corrugating adhesive composition comprised of a ketone-formaldehyde condensate which is treated with selected sulfite salts provides good water-resistance and viscosity stability while also providing low levels of free-formaldehyde content.
The water-resistant, viscosity stable, alkaline curing, starch-based corrugating adhesive composition of this invention comprises:
A. from about 10 to 40% by weight based on the total weight of the adhesive, of starch;
B. from about 0.3 to 5% by weight, based on the total weight of the starch, of an alkali;
C. from about 0.3 to 12% by weight, dry basis, based on the weight of starch, of a crosslinking additive prepared by reacting a ketone and formaldehyde in a molar ratio of 1 mole of ketone to about 1.5 to 6 moles of formaldehyde under aqueous alkaline conditions at about 20 to 80xc2x0 C. to obtain a water-soluble ketone-formaldehyde condensate containing about 1 to 4% by weight of unreacted formaldehyde and wherein from about 2 to 6.5 parts by weight of a water-soluble alkali metal or alkaline earth metal sulfite per part by weight of unreacted formaldehyde is added to the condensate to react with the unreacted formaldehyde present and allowing the reaction to proceed at about 20 to 45xc2x0 C. at a pH of about 5 to 9 until the unreacted formaldehyde is reduced to less than about 0.6% by weight based on the weight of the condensate, and
D. from about 54 to 89% by weight of water, based on the total weight of the adhesive.
The starch-based corrugating adhesive composition of this invention contains a selected crosslinking additive. This crosslinking additive is initially formed by reacting a ketone and formaldehyde under aqueous alkaline conditions at about 20 to 80xc2x0 C. to produce a water-soluble ketone-formaldehyde condensate containing about 1 to 4% by weight of unreacted (free) formaldehyde. The proportion of reactants will ordinarily range from about 1.5 to 6 moles of formaldehyde to about 1 mole of ketone and preferably about 2 to 4.5 moles of formaldehyde to 1 mole of ketone. In preparing the condensate, the reaction may be conducted under a nitrogen atmosphere if desired. It will be recognized that the alkalinity and reaction temperature employed must be no greater than is necessary to produce a water-soluble condensate which has not cured into an infusible product. Typically, the pH of the reaction mixture is maintained at about 8 to 12, preferably 9 to 11, by incremental addition of a solution of an alkaline agent such as sodium hydroxide. The reaction is monitored for formaldehyde content and when it is below about 3%, the product is cooled to about 25 to 45xc2x0 C. and neutralized with acid such as acetic acid, glacial acetic acid, and formic acid to a pH level of 4.8 to 8.2. The reaction time depends mainly on the temperature, alkalinity and desired solids content of the reaction mixture but is ordinarily such as to obtain a water-soluble ketone-formaldehyde condensate containing about 1 to 4% by weight of free-formaldehyde. The condensate will typically have a solids content of 40 to 65% by weight.
The formed ketone-formaldehyde condensate is then treated with an effective amount of selected sulfite salt to react with the unreacted or free-formaldehyde present. In order to provide good water resistance, reduce formaldehyde content and provide viscosity stability, it has been found that selected amounts of from about 2 to 6.5 and preferably from about 2.5 to 4.5 parts by weight of sulfite compound per part by weight of unreacted formaldehyde are especially useful. The condensate/sulfite mixture is then maintained at room temperature or at a temperature of about 20 to 45xc2x0 C., preferably about 25 to 35xc2x0 C., and at a pH of about 5 to 9, preferably about 6.5 to 8, until the unreacted formaldehyde is reduced to less than about 0.6%, preferably less than about 0.4% and more preferably less than about 0.1% by weight based on the weight of the condensate. This usually takes a fairly short period of time, e.g., 0.5 to 2 hours or more.
In making the crosslinking additive, the ketone may be any of the known monomers of the type including acetone, methylethyl ketone, acetophenone, benzophenone, cyclohexanone, etc. Acetone and its dimers, i.e., diacetone alcohol or mesityl oxide are especially useful with acetone being particularly preferred because of its cost, availability and reactivity. Sources of formaldehyde that can be used include gaseous formaldehyde, aqueous solutions of formaldehyde, trioxymethylene, hexamethylene tetraamine and paraformaldehyde.
Reduction in free-formaldehyde for the ketone-formaldehyde condensate is provided by adding a selected amount of sulfite salt. The sulfite salts which can be used are alkali metal and alkaline earth metal salts of sulfurous acid. More particularly, alkali metal and alkaline earth metal sulfites may be used and the term sulfites includes: sulfites, bisulfites, metabisulfites and disulfites. Sulfites might also be generated by, for example, the use of compounds such as sulfur dioxide which are converted to sulfurous acid in the presence of water and then to a sulfite. Preferred sulfites are the sodium sulfites and more preferably sodium meta-bisulfite or sodium bisulfite. The amount of sulfite compound necessary to attain good water resistance and viscosity characteristics as well as significantly reduced free-formaldehyde levels has been found to be a selected range of from about 2 to 6.5 and preferably 2.5 to 4.5 parts by weight of sulfite compound per part by weight of unreacted formaldehyde.
The corrugating adhesive composition of this invention is comprised of starch, water, alkali, the selected low formaldehyde crosslinking additive as described herein, and optionally borax. The starch component, which may be the ungelatinized starch and/or gelatinized carrier starch portion of the adhesive composition herein may be selected from any of the several starches, native or converted, heretofore employed in starch corrugating adhesive compositions. Suitable starches include, for example, those starches derived from corn, potato, waxy maize, tapioca, sorghum, wheat, as well as high-amylose starches, i.e., starches which contain 30% or more by weight of amylose, and the various derivatives of these starches. Hence, among the applicable starches are included the various starch derivatives such as ethers, esters, thin-boiling types prepared by known processes such as mild acid treatments, oxidation, etc. and those derivatives of these starches which have high amylose contents. Preferred starches are those typically employed in comugating adhesives of the alkaline type.
The starch content of the adhesive can vary considerably depending on several factors such as the intended end-use application of the adhesive and the type of starch used. The total amount of starch employed, including gelatinized and ungelatinized portions of starch (or in the case of xe2x80x9cno-carrierxe2x80x9d adhesives, the total starch content), ordinarily will be in the range of about 10 to 40% by total weight of the adhesive and preferably 18 to 35%.
The remainder of the adhesive composition is composed of about 0.3 to 5% of an alkali such as sodium hydroxide, based on total weight of starch, about 0.3 to 12% on dry basis, preferably 1 to 5%, of the low formaldehyde crosslinking additive as described herein, based on total weight of starch, and about 54 to 89% of water, based on total weight of the adhesive.
If desired, small amounts of borax or other boron containing salts, up to about 5% based on the total weight of starch, may be added to the adhesive to improve the tackifying properties thereof.
The alkali (base) employed herein is preferably sodium hydroxide; however, other bases may be employed in partial or full replacement of the sodium hydroxide and include, for example, alkali metal hydroxides such as potassium hydroxide, alkaline earth hydroxides such as calcium hydroxide, alkaline earth oxides such as barium oxide, alkali metal carbonates such as sodium carbonate, and alkali metal silicates such as sodium silicate. The alkali may be employed in aqueous or solid form.
In addition to the essential ingredients of the adhesive composition herein, any conventional non-chemically functional additives may be incorporated into the adhesive in minor amounts, if desired. Such additives include, for example, wetting agents, proteins, plasticizers, solubilizing agents, rheology modifiers, tackifiers such as borax, water conditioners, penetration control agents, peptizers such as urea, gelatinization temperature modifiers, inert fillers such as clay and finely ground polymers, thickeners such as inorganic colloidal clays, guar, hydroxyethyl cellulose, alginates, polyvinyl alcohol, polymers of ethylene oxide and the like, and emulsions such as polyvinyl acetate.
Additionally urea compounds such as urea and dihydroxyethylene urea may be added to the crosslinking additive and the adhesive composition to provide further improved stability characteristics, particularly to maintain low free-formaldehyde levels over time. Such urea compounds may be added in an effective stabilizing amount or an amount of about 0.25 to 10, preferably about 2.5 to 7.5% by weight, based on the weight of the condensate.
Further description regarding the acetone-formaldehyde crosslinking additive and the corrugating adhesive composition may be found in U.S. Pat. No. 5,247,066 issued to J. Schoenberg, et al. on Sep. 21, 1993 and which is incorporated by reference herein.
In the preparation of the adhesive composition herein, a portion of the total starch required in the adhesive is gelatinized in water with caustic soda to form the carrier, which is then slowly added to a mixture of raw starch, water and optionally borax. The crosslinking additive may be added to the raw starch mixture or to the final adhesive mixture as desired. While this description of the corrugating adhesive composition is directed to a composition comprising a carrier starch and a raw starch, it may also include a no carrier composition having just a single starch component comprising an ungelatinized starch which upon subsequent treatment with alkali becomes partially swollen.
The adhesive thus obtained can be used to bond single- or double-faced boards using any equipment which is presently employed for the preparation of corrugated board. The adhesive is maintained at a temperature preferably between 25xc2x0 and 55xc2x0 C. before its application to the protruding tips of the fluted paper strip. The actual application may be accomplished by the use of glue rolls which are ordinarily employed in most corrugating machines, or one may, if desired, utilize other application methods which may be able to achieve a different distribution of adhesive. Following the application of the adhesive to the fluted paper strip, the latter is then brought into immediate contact with the facing board under the influence of heat and pressure, as is well known in the art. A double-faced board may be subsequently prepared by bringing a second facing in contact with the open fluted surface of the single-faced board by the usual procedures.
Viscosity stability is a desired characteristic of the corrugating adhesive composition of this invention. While viscosity can vary to some degree depending on application conditions and materials used, the term xe2x80x9cviscosity stabilityxe2x80x9d may be defined as the ability to maintain a minimal change in the viscosity of the corrugating adhesive. This is the change in viscosity of the corrugating adhesive composition with crosslinking additive relative to the viscosity of the corrugating adhesive without the crosslinking additive. Typically, this will be a change in viscosity of the corrugating adhesive of less than about 70% and more particularly less than about 50% after 1 hour.